The invention disclosed herein relates generally to circuitry for providing an interface between sensors and a data bus, and more particularly to such circuitry capable of accepting inputs from both current sourcing and current sinking type sensors.
Programmable controllers are being increasingly used in a variety of industrial and other applications. Such controllers typically employ one or more microprocessors which must receive data signals having predetermined characteristics from a data bus. Signals on the data bus may be derived from a variety of sensors and other devices which produce various types of output signals whose characteristics generally do not meet the microprocessor data input signal requirements. Therefore, a suitable interface circuit must be used between each sensor or other input device and the data bus.
Two types of sensors are common. These comprise current sinking sensors which vary the current supplied by a current source in accordance with a sensed parameter, and current sourcing sensors which supply a variable current depending on a sensed parameter.
A variety of interface circuits are known for connecting either type of sensor to a data bus. Interface circuits for programmable controllers are also known which are provided with a selector switch permitting the user to select the proper interface circuit operating mode for either a current sinking or current sourcing input. Under this approach the installer is required to determine the transducer output type. The installer may or may not have the electronic capability and familiarity to correctly make this determination.
A similar potential problem occurs when a sensor is changed due to either a malfunction or changing application requirements. In this situation, the problem may be somewhat aggravated because the person doing the maintenance or modification work may be someone other than the original installer, and may be even less familiar with the types of sensor outputs.
Yet a further problem may occur in the common situation in which a particular application requires sensors of both types. Conventionally, interface circuits are available in modules of four or eight channels per module. If an application requires sensors of both types in other than multiples of four or eight sensors of each type, the cost of and a space required by the interface modules will be unnecessarily increased.
The foregoing problems or inconveniences are eliminated by the applicant's invention which makes each interface channel capable of indiscriminately accepting inputs from either current sourcing or current sinking type sensors or other input devices. Thus, the installer is not faced with the problem of selecting a proper interface channel for a particular input device, and may not even be required to know the type of the input device. In addition, utilization of the interface modules is maximized since separate modules are not required for current sourcing and current sinking type input devices.